Solvents capable of dissolving poly(amic-acid)s as typical polyimide precursors are disclosed in Journal of Polymer Science, Macromolecular Reviews, vol.11, 1976 (Table 2 on page 164). In this table, N,N-dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP), hexamethylphosphoramide (HMPA), N-methylcaprolactam, dimethyl sulfoxide (DMSO), N-acetyl-2-pyrrolidone, N,N-dimethylacetamide (DMAc) and the like are listed as illustrative examples of such type of solvents. They are generally called "aprotic polar solvents" and have a high polarity with a dipole moment of about 3.0 debyes or more. For example, the just described typical poly(amic-acid) solvents DMAc, NMP, DMSO and DMF respectively have dipole moments of 3.7, 4.1, 4.3 and 3.9 debyes. The above-cited reference discloses that these solvents can be used not only as solvents for dissolving poly(amic-acid)s but also as polymerization solvents when a poly(amic-acid) is produced by polymerizing a diamine with a tetracarboxylic dianhydride.
In addition, the above-cited reference also discloses that a polyimide film can be obtained by dissolving a poly(amic-acid) in such an aprotic polar solvent and then distilling off the solvent from the solution to effect imidation and that a polyimide coating can be obtained by applying such a solution on a base material and then distilling off the solvent from the coated solution to effect imidation.
U.S. Pat. No. 4,238,528 discloses polyimide precursor solutions which comprise combinations of polyimide precursors, solvents [for example, NMP/acetone, NMP/cellosolve, NMP/xylene, NMP/toluene and (2-ethoxyethanol)-cellosolve/acetone] and nonionic fluorocarbon surface active agents.
JP-B-3-4588 discloses a polyimide precursor solution which comprises an aprotic polar organic solvent (selected from DMAc, NMP, DMSO and DMF), a polyimide precursor, a halogenated aliphatic hydrocarbon, a specified organic solvent and an organic silane. (The term "JP-B" as used herein means an "examined Japanese patent publication")
In addition, IBM Technical Disclosure Bulletin (vol.20, No.6, p.2041, November, 1977) discloses a polyimide precursor solution which is formed by mixing pyromellitic dianhydride/DMSO with diaminodiphenyl ether/NMP.
However, each of these aprotic polar solvents usually used as solvents of polyimide precursors, such as DMAc, NMP, DMSO, DMF and the like, strongly associates with a polyimide precursor which is the solute in the resulting solution because of the high dipole moment of the solvent. Such a phenomenon has been reported, for instance, in Journal of Polymer Science (A-1, vol.4, pp.2607-2616, 1966), Journal of Polymer Science (A, vol.25, pp.2005-2020, 1987), Journal of Polymer Science (A, vol.25, pp.2479-2491, 1987), Kogyo Kagaku Zasshi (vol.71, No.9, pp.1559-1564, 1968) and ANTEC '91 Abstract (pp.1742-1745). Because of the strong solvent/solute solvation, these prior art solvents cause various problems when polyimide films, polyimide coatings and the like are produced. In addition, since each of these solvents has a high dipole moment as has been disclosed in JP-B-3-4588, they have a high surface tension and show high viscosity levels which also cause various problems.
Thus, when a polyimide precursor is dissolved in a prior art aprotic polar solvent such as DMAc, NMP, DMSO, DMF or the like, the resulting solution shows a poor storage stability, and working conditions cannot be kept constant at the time of the production of moldings or coatings from the solution, in addition to difficulty in removing the solvent from the solution when moldings or coatings are produced. In the prior art process, a polyimide precursor solution is produced by a so-called "low temperature solution polymerization" in which a diamine (such as diaminodiphenyl ether) is allowed to undergo a polymerization reaction with a tetracarboxylic dianhydride such as pyromellitic dianhydride in an aprotic polar solvent. It has been believed that a polymerization solvent to be used in such a case must be a water-free good solvent which can dissolve monomers in a high concentration. Since hydrolysis of the acid anhydride progresses when water coexists in the aprotic polar solvent, it is necessary to carry out the polymerization reaction in a strictly water-free system which requires complex chemical reactors. In addition to the above, there are many other problems with regard to the prior art polar solvents such as: high production cost due to cost of the solvents; insufficient electric properties of the produced moldings or coatings because of the high amount of residual solvent; and insufficient uniformity of coatings or insufficient adhesiveness of coatings to a base material when a film or a coating is produced. Also, the solvent remaining in a molding is decomposed when the temperature of the molding is increased at the time of its use, thus entailing the generation of toxic carbon monoxide.